Severe accidents in nuclear reactors have been investigated in many countries since 1979 and the Three Mile Island accident.
To reduce the probability of such accidents and evaluate their consequences, the French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) participates in international research programs about severe accidents, including experimental programs and code development.
The behaviour of molten corium in the lower plenum of a reactor vessel is a topic for which current understanding is not sufficient to be able to predict accurately the possible cooling of materials or the failure of the vessel. The complexity arises from the combination of high temperatures (above 2000 K), a variable mixture composition (oxides, metals) and the presence of several phases (solid, non miscible liquids and gas).
In the present study, a comprehensive modelling to predict the stratification of oxides and metals in a corium pool is presented. It consists in a series of models that take into account both thermochemical and thermalhydraulic aspects. The model is designed to be used on a 2D/3D meshing. Therefore, the corium pool is assumed to be non homogeneous in temperature and composition. Only local thermochemical equilibrium is assumed (in each mesh).
A simplified thermochemical model provides the equilibrium compositions of the oxide and metal phases. The densities depend on the local compositions and temperature and the relative motion of the two phases is calculated with a drift flux model. This provides the velocities of materials in the pool and, therefore, the global transport of species. Fission products are also transported, assuming, for each fission product, a pre-defined local equilibrium distribution between oxide and metal phases.
The model is implemented in a test-version of ICARE/CATHARE V2.0, a severe accident code developped by IRSN. A one-dimensional transient with successive steel additions is presented to demonstrate the existence of transient "3-layers" configuration. It corresponds to a global non-equilibrium situation which may remain for a significant time, depending on species transport process across the oxide pool. A reactor configuration is also presented, to study the effects of the progressive melting of lower plenum steel structures in a two-dimensional corium pool.
(1) : Post-Doc IRSN